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Applying Benthic Data: Mapping Contaminated Sediments

Map of contaminated sediments on the Palos Verdes Shelf off the coast of Los Angeles, California.

Map of contaminated sediments on the Palos Verdes Shelf off the coast of Los Angeles, California.
Courtesy: Environmental Protection Agency

Palos Verdes Shelf

Throughout the 1950s, 1960s and early 1970s, chlorinated pesticide (DDT) and polychlorinated biphenyl (PCB) were transported through the Los Angeles County sewer system and deposited into the ocean sediments off the Palos Verdes Peninsula. Although DDT was no longer discharged through the sewer system by the early 1970s, the contamination persists. Sediment mixing caused by benthic organisms and physical processes still transports buried contaminants into surface sediments.

DDT and PCBs are ingested by organisms and transferred up the food chain. These contaminants accumulate in the tissues of fish, birds, and marine mammals. Birds such as brown pelicans and bald eagles have suffered reproductive losses due to eggshell thinning caused by DDT. Humans are at risk as well through consumption of commercial fish such as white croaker, kelp bass, rockfish, sheepshead, surfperches, and sculpin. Fish contaminated with DDT and PCBs can increase cancer risks, harm the liver, and affect the central nervous system of people who eat them.

As part of an effort to determine the extent of contamination and possibilities for remediation, the United States Geological Survey (USGS) conducted surveys on the Palos Verdes Shelf from 1992 to 1994. The goal of this work was to map the distribution of DDT and PCBs on the shelf and to predict future distributions if no remediation occurred. Multiple techniques were used to collect data for mapping.

Side-scan sonar surveys were used to measure the thickness and distribution of the contaminated sediment deposit. Bottom photography, via a benthic camera-sled, showed surface features and sediment transport on the seafloor. Box cores collected undisturbed samples of contaminated sediments. These samples were analyzed for a compound known as DDE (a degradation product of DDT used to approximate DDT concentrations) and PCBs. Vibracores, which can penetrate deeper into the sediment than a box core, were used to characterize the sediment layers underneath the contaminated sediments. In addition to these survey methods, historical coring data from the Los Angeles County Sanitation District were also used.

Map of contaminated sediments on the Palos Verdes Shelf off the coast of Los Angeles, California.

Click to see a full map of DDT concentrations. View Image

The survey data were used to create 3-dimensional (3-D) maps of the contaminated sediment deposit showing DDE distributions for various time periods.

In addition to 3-D distribution maps, data from this study were used to determine depths of maximum contamination throughout the site, to calculate mass of contaminants per unit area, and to calculate total contaminant mass. Results showed that the layer of contaminated sediments was approximately 20 to 60 centimeters thick. The contaminated sediment deposit was spread over a 40 square kilometer area, with 9 million cubic meters of contaminated sediment. At least 100 tons of DDT were determined to be present. Furthermore, comparisons of contaminated sediment volume over time showed that contaminant volumes had not decreased significantly over a 16-year period. This indicates that natural, physical, and biological mixing processes are not sufficient to decrease the environmental hazards of DDT and PCB contamination on the Palos Verdes Shelf.

The Palos Verdes Shelf project illustrates the importance of mapping contaminated sediments. These maps allowed for comparisons over specific time intervals — showing that remediation efforts are needed to reduce contamination in this area. Sediment contamination maps also provided the basis for estimates of contaminated sediment volume. These estimates were used by the Environmental Protection Agency (EPA) to evaluate potential remediation options, including sediment capping. Volume calculations allow researchers to determine the size, location, volume, and cost of a possible sand cap. Currently a pilot project is underway to evaluate capping as an effective remediation tool on the Palos Verdes Shelf.

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